Related Experiment Video
Updated: Apr 28, 2026
Convection Enhanced Delivery of Optogenetic Adeno-associated Viral Vector to the Cortex of Rhesus Macaque Under Guidance of Online MRI Images
Published on: May 23, 2019
Axon diameters and conduction velocities in the macaque pyramidal tract
L Firmin1, P Field2, M A Maier3
1Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, United Kingdom; Research Department of Cell and Developmental Biology, University College London, United Kingdom; FR3636 Centre National de la Recherche Scientifique/Université Paris Descartes and Université Paris Diderot, Sorbonne Paris Cité, France; and.
Small axons in the corticospinal tract are numerous but poorly understood. This study reveals a significant mismatch between anatomical and physiological data, with small, slow axons being underrepresented in recordings.
Area of Science:
- Neuroscience
- Motor control
- Axonal physiology
Background:
- Small axons constitute the majority of fibers in the corticospinal tract.
- The function and physiological properties of these small axons are largely unknown due to identification challenges.
Purpose of the Study:
- To investigate the mismatch between anatomical and physiological measures of corticospinal tract axons.
- To characterize the distribution of axon diameters and conduction velocities in macaque corticospinal neurons.
Main Methods:
- Electron microscopy was used to quantify axon diameters in the medullary pyramid of macaque monkeys.
- Electrophysiological recordings (antidromic and orthodromic) were performed to assess conduction velocities and latencies of corticospinal neurons.
Main Results:
- Approximately 52% of sampled axons had diameters smaller than 1 μm, with 14% smaller than 0.50 μm.
- Electrophysiological recordings were heavily biased towards large, fast-conducting axons (>3 μm, >18 m/s).
- Small, slow-conducting axons (<1 μm, <6 m/s) were likely absent from electrophysiological recordings.
Conclusions:
- There is a substantial underrepresentation of small, slow-conducting corticospinal axons in physiological studies.
- The identity, location, and function of the majority of small corticospinal neurons remain undetermined.
